1) Field of the Invention
The present invention relates to a portable terminal that communicates with base stations by use of an orthogonal frequency division multiplexing (OFDM) signal, a communication system including the portable terminal, and a communication method.
2) Description of the Related Art
In a conventional OFDM communication system, the OFDM signal includes a guard time of a predetermined length to suppress the deterioration of the OFDM signal that is caused by the influence of delayed waves. It is also known in the conventional art to determine the length of the guard time based on the delay time by a multipath propagation (For example, see Japanese Patent Application Laid-open No. 2002-374223).
In the OFDM communication between a portable terminal and a base station, a handover occurs as the portable terminal moves. It is known that OFDM signals sent from two base stations become degraded due to a difference in timing between the OFDM signals, during handover.
In the conventional OFDM signal communication system, however, no countermeasure has been developed or introduced against transient deterioration of the OFDM signals during handover. There is another problem of the conventional art that changing the length of the guard time with a change in the propagation environment requires stringent specifications for synchronization of timing for sending signals to the base station. Further, a communication system involving a spread spectrum modulation has a problem that channel throughput decreases when the length of the guard time is changed.